The great French philosopher-statesman Alexis de Tocqueville once called for "a new political science for a new world". It is a good motto for liberal social democrats who hoped for a realignment of the left and centre-left, and now confront a baffling counter-realignment of the centre-right. To understand the new politics of the 2010s we must jettison the ideological clutter of the last half-century. The first step is to take a hard, unsentimental look at the presumptive partners in the progressive alliance that never was, and at the failures of nerve and imagination that led them to duck the challenge it posed.

Labour's failures are the most blatant. Had Gordon Brown wholeheartedly embraced the constitutional reform agenda he toyed with immediately before and after becoming prime minister, a progressive alliance might have been built three years ago. I don't know what went on in the cavernous recesses of his soul, but I think he was telling the truth when he said he wanted a new constitutional settlement. The trouble was that he couldn't face down the mastodons of the Labour movement and overcome the deadly mixture of prolier-than-thou posturing, top-down statism and jealous tribalism that is part and parcel of the Labour culture.

Fundamental to any progressive alliance worthy of the name would be a politics of power-sharing, tolerance and republican self-government. Such a politics is light years from Labour's inherited instincts. There are pluralists in Labour's ranks; the pressure group Compass contains many. But most Labour people are power-hoggers by nature, not power-sharers. Labour could not have taken part in a genuine progressive alliance without a cultural revolution. Of that there was (and is) no sign.

So far, so Nick Clegg. But the Liberal Democrats failed too; and their failures were at least as deeply rooted. Like the Liberals before them, they are Janus-faced. In part of their minds they are an anti-system party. They stand for people power and community politics; they thrill to the saying, "Power is like muck – no good unless it be spread". Through long years in the wilderness they have stuck to the guns primed by the likes of Tom Paine and John Stuart Mill.

But at the same time they yearn desperately for respectability. They want to kick the establishment, but they also want to belong to it. Like Brown's, the inner recesses of Nick Clegg's soul are a mystery to me. But there is not much doubt that one of the reasons why he and his colleagues acted as they did is that a Liberal Democrat-Conservative coalition offered them the best route to their secret heart's desire: establishment status and acceptance. Clegg's incredulous grin as he stood next to Cameron in their first press conference said it all.

For would-be realigners, Labour and the Liberal Democrats are both broken reeds. Compass or no Compass, the Labour party is not going to change in a hurry. The Liberal Democrats have undoubtedly changed, but in the wrong way. They have lost their reason for existing. There was point in voting Liberal Democrat when they could claim to be the party of insurgent democracy. There is no point in voting for them merely because they have buttressed Cameron's Whiggish statecraft.

In day-to-day terms, this government is likely to be a great improvement on its dysfunctional and illiberal predecessor. Nearly all Clegg's proposed political reforms deserve two cheers; many deserve a rousing three. But that is not the point. The Liberal Democrats may or may not be swallowed up organisationally, but they are already being swallowed up ideologically. They are turning into a second Whiggish establishment party. And there is no room for two.

But though realignment as Westminster politicians have understood it is now a dead duck, a richer and deeper realignment is desperately needed. For the last two years we have been living through the third great capitalist crisis of modern times; and it is not over yet. The neoliberal paradigm that has dominated policy-making throughout the developed world, not least in the institutions of global economic governance, has been turned inside out. Markets, we have discovered (or rediscovered), do not always know better than governments. Private greed does not procure public benefits. The lords of creation in the hedge funds and investment banks are not wealth creators. They are wealth destroyers. A rising tide does not invariably float all boats.

The self-regulating market of neoliberal economic theory is a phantom, whose pursuit led to a shameful increase in inequality and eventually to a catastrophic fall in employment and output. The newly untamed capitalism of the last 30 years has not been driven by "rational economic actors": the "rational economic actor" is another phantom. It has been driven by stampeding herds of electronic gamblers. It is not only monstrously unjust, it is also unsustainable – not only economically, but politically, environmentally and, above all, morally.

Yet the implications have not sunk in. In Washington, London and the capitals of the eurozone, the hunt is on for a tarted-up version of business as usual, radical enough to seem new, but conservative enough to keep the essentials of the old show on the road. Hayek and Friedman have been toppled from their perches; Keynes has returned to his. Tougher regulation, banking reforms, quantitative easing and even bank nationalisations have been the order of the day; some still are.

However, this is an elaborate exercise in rearranging the deckchairs on the Titanic. As Will Hutton once pointed out, the point about the Keynesian revolution is that it wasn't revolutionary. Keynes was a small-c conservative, not a radical. He wanted to save capitalism, not to supersede or even to transcend it. And almost by definition, he couldn't foresee the accelerating environmental crisis of our day. There will be no salvation from his quarter: of the great economists of the past, Marx is a better guide than Keynes to the turbulent, masterless capitalism of today.

But none of the economic gurus of old days is of much help now. The one certainty is that we can't continue indefinitely on our present path – and by "we" I don't just mean British politicians and voters; I mean the human race. Sooner or later the crisis-haunted capitalist merry-go-round will have to stop. The great question for our time is not how and when to cut the fiscal deficit, or calm the markets, or curb the bonus culture, or tax transnational financial transactions – pressing as all these questions are. It is how to halt the merry-go-round before it is too late: how to switch from an unjust and unsustainable economic order to a just and sustainable one.

No single thinker, party or school of thought offers a complete answer, or anything like it. Answers will have to be hammered out in open-minded dialogue, between all those who accept that tinkering is not enough, across the lines of party and creed. The need, in fact, is for a realignment of the mind, socialist in economics and republican in politics. In such a realignment the Green movement must surely have a central place, along with radicals and dissenters from all parties and none. Caroline Lucas, over to you.